The field of the disclosure relates to arrays of solid state detectors that work together to determine a quantity of particles or photons emitted by the sample. Other aspects of the disclosure include a detector and methods for counting secondary electrons with an array of solid state devices and a method for reducing the noise from the data derived from the array of solid state devices.
The scanning electron microscope (SEM) is a basic tool in materials science, electronics, energy, medical science and other disciplines, vital to the national and public well-being. Secondary electron (SE) topographic imaging is the most common mode of operation in the SEM. Its current limitations, however, prevent advances that otherwise could be accomplished in SE imaging.
SE imaging is most commonly performed using a scintillator-photomultiplier based Everhart-Thornley (E-T) detector, or some variation thereof. An E-T detector images all sides of features with the contrast originating from SE collection differences due to subtended angle with detector, tilt, edge effect, and shadowing. FIG. 1 depicts the operational features of a traditional Evanhart-Thornley (E-T) scintillator-photomultiplier to form an image from SEs escaped from the sample surface layers. As the electron beam rasters the sample pixel by pixel, the E-T detector collects the low-energy (5-50 eV) SEs. Furthermore, the Faraday Cage is an integral part of an E-T detector. The detector surface maintained at +10 kV further accelerates the SEs. The scintillator layer emits photons upon being impinged upon by the SEs. The photons travel down the light pipe, hit the photocathode, convert into electrical signal, and is amplified. Analog to digital converters (ADC) converts the signal to digital pixels.
The numbers of components in the detector, and the signal processing are subject to sources of noise, distortion, and non-linearity. Hence, measurement using E-T signals has reduced quantitative utility [Reimer, 1985] at high resolution. In FIG. 2 a SEM trace was performed using an electron beam to scan across an array of identical structures [Joy, 2012], recorded from an E-T detector. As the beam reaches the left edge of the first feature, the signal rises sharply. As a result the analog feedback loop starts to reduce the system gain and DC-offset. As the beam keeps scanning on the other side of the feature, the signal level falls but decays even faster because the feedback loop is still driving the signal down. As a result, the signal is highly distorted by the feedback and exhibits increased DC-offset, suppressed dynamic range (see FIG. 2), varying “dark” level from pixel to pixel, poor signal-to-noise ratios (SNR) and non-linear relation between contrast and SE collection [Joy, 2012; Merli, 1995; Kazemian, 2007; Postek, 2012; Bogner, 2007; Oho, 2007; Joy, 1992; Isaacson, 1977; ITRS, 2011: Metrology Challenges]. The DoE Electron Scattering Workshop [DOE_BES, 2007] has identified broad research needs in electron microscopy. Similarly, the Semiconductor Industry has identified metrology challenges [ITRS, 2011].